Reverse replay of behavioural sequences in hippocampal place cells during the awake state

The hippocampus has long been known to be involved in spatial navigational learning in rodents(1,2), and in memory for events in rodents(3,4), primates(5) and humans(6). A unifying property of both navigation and event memory is a requirement for dealing with temporally sequenced information. Reactivation of temporally sequenced memories for previous behavioural experiences has been reported in sleep in rats(7,8). Here we report that sequential replay occurs in the rat hippocampus during awake periods immediately after spatial experience. This replay has a unique form, in which recent episodes of spatial experience are replayed in a temporally reversed order. This replay is suggestive of a role in the evaluation of event sequences in the manner of reinforcement learning models. We propose that such replay might constitute a general mechanism of learning and memory.